1. Related Field
The present invention relates to a method for increasing the resolution in additively manufactured three-dimensional articles according to the preamble of claim 1.
2. Description of Related Art
Freeform fabrication or additive manufacturing is a method for forming three-dimensional articles through successive fusion of chosen parts of powder layers applied to a worktable.
Such an apparatus may comprise a work table on which said three-dimensional article is to be formed, a powder dispenser, arranged to lay down a thin layer of powder on the work table for the formation of a powder bed, an energy beam for delivering energy to the powder whereby fusion of the powder takes place, elements for control of the energy given off by the energy beam over said powder bed for the formation of a cross section of said three-dimensional article through fusion of parts of said powder bed, and a controlling computer, in which information is stored concerning consecutive cross sections of the three-dimensional article. A three-dimensional article is formed through consecutive fusions of consecutively formed cross sections of powder layers, successively laid down by the powder dispenser.
When an energy beam in the form of an electron beam hits the powder, a charge distribution develops around the electron target area. Desirably, this charge will be led through a produced part of the article to be made and/or the powder bed towards ground. If the charge distribution density exceeds a critical limit, an electrical field having field strength above a predetermined level will develop around the position where the beam is radiating. The electrical field having electrical field strength above the predetermined level will be referred to as Emax. An electrical field will cause the powder particles to repel each other such that particles leave the uppermost surface layer of the particle and create a distribution of particles floating above the surface. The floating particles resemble a cloud positioned above the surface. When the electrical field has field strength above Emax, the electrical field, i.e. the particle cloud or smoke of powder, will influence the resolution of the device in a negative way. This is partly due to the fact that the particles in the particle cloud will diverge the electron beam. When the electrical field has field strength below Emax, the electrical field, i.e. the particle cloud, will not influence the resolution of the device in a significant way. A field strength below Emax is thus desirable.
Since the particles are charged they will seek a ground contact and thereby some may leave the cloud and will then contaminate different parts of the device being positioned inside the vacuum chamber. A result of such a critical electrical field is that the structure of the powder surface will be destroyed.
One solution to the problem of avoiding charging of powder is disclosed in WO 2008/147306. In said document the amount of ions present in close vicinity to the position where the electron beam radiates the powder material is controlled. This is according to one example embodiment performed by introducing a supplementary gas into the vacuum chamber, which is capable of producing ions when irradiated by the electron beam.
The problem with said solution is that the supplementary gas increases the electron beam spot dimension and thereby affects the resolution of the additive manufacturing process in a negative way.